Method and system for vacuum control

ABSTRACT

Methods and systems are provided for controlling and coordinating control of a post-catalyst exhaust throttle and an EGR valve to expedite catalyst heating. By closing both valves during an engine cold start, an elevated exhaust backpressure and increased heat rejection at an EGR cooler can be synergistically used to warm each of an engine and an exhaust catalyst. The valves may also be controlled to vary an amount of exhaust flowing through an exhaust venturi so as to meet engine vacuum needs while providing a desired amount of engine EGR.

FIELD

The present description relates to systems and methods for expeditingheating of an exhaust catalyst, in particular, during an engine coldstart.

BACKGROUND AND SUMMARY

Vehicle engine systems may include various vacuum actuators, such asvehicle brakes, that utilize vacuum as an actuation force. The vacuum istypically supplied by the engine through a connection to the intakemanifold, which is at sub-barometric pressure when the intake throttleis partially closed and regulating the airflow into the engine. In someexamples, engine-driven or electrically-driven vacuum pumps may be usedto supplement intake manifold vacuum during conditions (e.g., during anengine cold-start and warm-up) when intake manifold vacuum does notprovide sufficient vacuum for operating all the various vacuumactuators. However, engine-driven vacuum pumps may disadvantageouslyreduce fuel economy, while electrically-driven vacuum pumps may lackdurability while being expensive, heavy, and noisy. In still otherexamples, ejectors positioned at different locations in the enginesystem may be used to provide at least a portion of the vacuum used bythe actuators. In particular, flow of air and/or exhaust gas through theejector may be harnessed to generate vacuum for use by the vacuumactuators. However, since vacuum generation at an ejector is related toflow through the ejector, changes in air flow or exhaust flow, such asduring boosted operating conditions or during exhaust gas recirculation,may affect vacuum generation at the ejector.

The inventors have recognized the issues with these options and offersystems and methods for more reliable vacuum generation at differentengine operating conditions, with the further advantage of expeditingcatalyst warming. In one embodiment, a method for an engine includes,while recirculating an amount of exhaust gas to an engine intake,closing a post-catalyst exhaust throttle to increase vacuum generationat an exhaust ejector, and closing an EGR valve to maintain the amountof exhaust gas recirculation.

The presented approach may offer several advantages. For example, rapidcatalyst heating may be attained. By rapidly heating the catalyst,exhaust emissions during engine cold starts may be reduced.Additionally, vacuum may be generated in copious amounts during the verycondition (catalyst heating) when it is less available via the intakemanifold. This is accomplished by directing exhaust through the ejector,thus reducing the need for engine-driven or electrically-driven vacuumpumps to supplement intake manifold vacuum. Further, by adjusting theEGR valve in concert with the exhaust throttle, vacuum may be generatedin the presence of EGR flow, as well as while exhaust flow through anEGR passage fluctuates. At the same time, a desired EGR rate and enginedilution can be maintained so that engine performance is not degraded.

It should be understood that the summary above is provided to introducein simplified form a selection of concepts that are further described inthe detailed description. It is not meant to identify key or essentialfeatures of the claimed subject matter, the scope of which is defineduniquely by the claims that follow the detailed description.Furthermore, the claimed subject matter is not limited toimplementations that solve any disadvantages noted above or in any partof this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic depiction of an engine system.

FIG. 2 shows a high level flow chart illustrating a routine that may beimplemented for adjusting an exhaust backpressure valve and an EGR valveduring an engine cold-start so as to expedite engine warm-up andcatalyst activation.

FIG. 3 shows an example exhaust backpressure valve and EGR valveadjustment for expediting engine warm-up, according to the presentdisclosure.

FIG. 4 shows a high level flow chart illustrating a routine that may beimplemented for operating the engine system of FIG. 1 in variousoperating modes based on exhaust catalyst temperature, engine EGR needs,and engine vacuum needs.

FIG. 5 shows an example exhaust backpressure valve and EGR valveadjustment for meeting engine vacuum and EGR needs, according to thepresent disclosure.

DETAILED DESCRIPTION

Methods and systems are provided for expediting engine warm-up andcatalyst activation in a vehicle engine, such as the engine system ofFIG. 1. During an engine cold-start and warm-up, synergistic benefits ofincreased exhaust backpressure and increased heat rejection at an EGRcooler may be advantageously used to quickly raise an enginetemperature. A controller may be configured to perform a controlroutine, such as the example routine of FIG. 2, to throttle an exhaustvalve positioned downstream of an exhaust catalyst to raise an exhaustbackpressure while also closing an EGR valve to flow at least a portionof the throttled exhaust gas through an EGR cooler. The increasedbackpressure enables a rapid increase in engine temperature by trappinghot exhaust gas in engine cylinders, while flow of throttled exhaust gasthrough an EGR cooler enables a further increase in engine coolanttemperature via exhaust heat rejection at the EGR cooler. Thesynergistic combination enables faster activation of an exhaustcatalyst, while also addressing engine cold-start NVH issues. Thecontroller may also be configured to perform a control routine, such asthe example routine of FIG. 4, to operate and transition the enginesystem between various operating modes based on an engine heating needs,EGR needs, and vacuum needs. By adjusting one or more of the EGR valveand the exhaust throttle, EGR may be provided while heating an exhaustcatalyst and drawing vacuum at an exhaust ejector. Example valve andexhaust throttle adjustments are described at FIGS. 3 and 5.

FIG. 1 shows a schematic depiction of a vehicle system 106. The vehiclesystem 106 includes an engine system 108, including engine 100 coupledto emission control system 122. Engine 100 includes a plurality ofcylinders 130. Engine 100 also includes an intake 123 and an exhaust125. Intake 123 may receive fresh air from the atmosphere through intakepassage 142. Air entering intake passage 142 may be filtered by airfilter 190. Intake passage 142 may include an air intake throttle 182positioned downstream of an intake compressor 152 and an intake chargeair cooler 184. Intake throttle 182 may be configured to adjust the flowof intake gas (e.g., boosted intake air) entering engine intake manifold144. Exhaust 125 includes an exhaust manifold 148 leading to an exhaustpassage 145 that routes exhaust gas to the atmosphere via tailpipe 135.

Engine 100 may be a boosted engine including a boosting device, such asturbocharger 150. Turbocharger 150 may include intake compressor 152,arranged along intake passage 142, and an exhaust turbine 154, arrangedalong exhaust passage 145. Compressor 152 may be at least partiallydriven by turbine 154 via shaft 156. The amount of boost provided by theturbocharger may be varied by an engine controller. In some embodiments,a bypass passage controlled via a wastegate (not shown) may be coupledacross the exhaust turbine so that some or all of the exhaust gasesflowing through exhaust passage 145 can bypass turbine 154. By adjustingthe position of the wastegate, an amount of exhaust gas deliveredthrough the turbine may be varied, thereby varying an amount of boostdelivered to the engine intake.

In further embodiments, a similar bypass passage controlled via a bypassvalve (not shown) may be coupled across the intake compressor so thatsome or all of the intake air compressed by compressor 152 can berecirculated into the intake passage 142 upstream of compressor 152. Byadjusting the position of the compressor bypass valve, pressure in theintake system may be released during selected conditions to reduce theeffects of compressor surge loading.

An optional charge air cooler 184 may be included downstream ofcompressor 152 in the intake passage to reduce the temperature of intakeair compressed by the turbocharger. Specifically, after-cooler 184 maybe included upstream of intake throttle 182 or integrated into theintake manifold 144.

Emission control system 122, coupled to exhaust passage 145, includes acatalyst 170. Catalyst 170 may include multiple catalyst bricks, in oneexample. In another example, multiple emission control devices, eachwith multiple bricks, can be used. Catalyst 170 can be a three-way typecatalyst in one example. In other examples, catalyst 170 may be anoxidation catalyst, lean NOx trap, selective catalyst reduction (SCR)device, particulate filter, or other exhaust treatment device. Whilecatalyst 170 is arranged downstream of turbine 154 in the embodimentsdescribed herein, in other embodiments, catalyst 170 may be arrangedupstream of a turbocharger turbine or at another location in the engineexhaust passage without departing from the scope of this disclosure.

An exhaust throttle or backpressure valve 164 may be located in theexhaust passage, downstream of exhaust catalyst 170. In the embodimentsdescribed herein, controller 120 may control a position of exhaustthrottle 164 based on various engine operating conditions and parametervalues (e.g., engine cold start, stored vacuum level, shutdown, etc.).In other embodiments, the exhaust throttle, exhaust passage, and othercomponents may be designed such that the exhaust throttle ismechanically controlled as needed during various engine operatingconditions, without control system intervention. As elaborated withreference to FIG. 2, exhaust throttle 164 may be selectively closed bycontroller 120 during engine cold-start conditions to rapidly raise anexhaust pressure and temperature. By throttling the exhaust valve, alarger amount of hot exhaust gas can be trapped in an engine cylinder,further raising an exhaust temperature and expediting the downstreamexhaust catalyst reaching its activation temperature.

As such, the improvement in heat transfer to the engine and exhaustcatalyst via throttling of the exhaust can be attributed to at least twoeffects. First, the higher density of the (slower moving) exhaust gas,due to the higher pressure of the exhaust gas, improves heat transferper kilogram of the exhaust flow. Said another way, when throttled, thehigh temperature exhaust gas spends more time in contact with thecatalyst, the desired recipient of the heat. Further, the expansion toatmosphere post-catalyst (e.g., post an exhaust three-way catalyst)drops the temperature below ambient temperature, creating a heat pumpeffect. Consequently, substantially all the exhaust heat can berecovered without requiring the addition of a heat exchanger. Inparticular, by using a post-catalyst exhaust throttle, the time andtemperature that a given mass of exhaust gas is in contact with engineparts is substantially increased. This expedites the catalystactivation. It will be appreciated that while the depicted embodimentachieves post catalyst expansion of the exhaust via an exhaust throttle,in alternate embodiments, the same may be achieved via a post-catalystorifice in the engine exhaust passage 145.

Exhaust throttle 164 may be maintained in a fully open position (or wideopen throttle) during most engine operating conditions, but may beconfigured to close to increase exhaust backpressure under certainconditions, as will be detailed below. In one embodiment, exhaustthrottle 164 may have two restriction levels, fully open or fullyclosed. However, in an alternate embodiment, the position of exhaustthrottle 164 may be variably adjustable to a plurality of restrictionlevels by controller 120.

As detailed herein, adjustments of exhaust throttle position may affectair flow through the engine. For example, a fully closed exhaustthrottle may be conceptualized as a “potato in the tailpipe” whichrestricts exhaust flow, thereby causing an increase in exhaustbackpressure upstream of the closed exhaust throttle. This increase inexhaust backpressure leads to a direct increase in exhaust temperaturewhich may be advantageously used during selected conditions (e.g.,during an engine cold-start and warm-up) to expedite warming of exhaustcatalyst 170. In some embodiments, while closing the exhaust throttle,spark timing may be retarded to further elevate exhaust temperatures,thereby further expediting catalyst activation.

To compensate for the effects of exhaust throttle adjustment on engineair flow, one or more other engine components may be adjusted. As anexample, as the exhaust throttle closes, mass air flow may initiallydecrease, and thus an intake throttle (such as intake throttle 182) maybe opened to admit more air to the engine to maintain engine speed andreduce torque fluctuation. In this way, while the exhaust throttle isused to manage backpressure, airflow may be controlled to limit anengine output torque. As another example, spark timing may be adjusted(e.g., advanced) while the exhaust throttle is closed to improvecombustion stability. In some embodiments, valve timing adjustments mayalso be used (e.g., adjustments to an amount of valve overlap) inconjunction with throttle position adjustments to improve combustionstability. For example, intake and/or exhaust valve timings may beadjusted to adjust internal exhaust gas recirculation and increasecombustion stability.

Vehicle system 106 further includes a low-pressure EGR (LP-EGR) system161. LP-EGR system 161 includes an EGR passage 163 that couples exhaustpassage 145, downstream of exhaust catalyst 170 and upstream of exhaustthrottle 164, with air intake passage 142, upstream of compressor 152.An EGR cooler 162 arranged in EGR passage 163 cools exhaust gas flowingthere-through, as will be detailed below. A position of EGR valve 159,located in EGR passage 163 on the intake passage side of EGR cooler 162,may be adjusted by controller 120 to vary an amount and/or rate ofexhaust gas recirculated from the exhaust passage to the intake passagevia the LP-EGR system. In some embodiments, one or more sensors may bepositioned within LP-EGR passage 163 to provide an indication of one ormore of a pressure, temperature, and air-fuel ratio of exhaust gasrecirculated through the LP-EGR passage. For example, temperature sensor118 may be coupled to an outlet (on the intake passage side) of EGRcooler 162 and may be configured to provide an estimate of an EGR cooleroutlet temperature. As elaborated below, during an engine cold-start andwarm-up, an opening of exhaust throttle 164 may be adjusted based on theEGR cooler outlet temperature to expedite heating of an enginetemperature. Exhaust gas recirculated through LP-EGR passage 163 may bediluted with fresh intake air at a mixing point located at the junctionof LP-EGR passage 163 and intake passage 242. Specifically, by adjustinga position of EGR valve 159, a dilution of the EGR flow may be adjusted.

As such, when EGR valve 159 is closed, at least a portion of exhaust gasmay be directed through EGR cooler 162. As elaborated with reference toFIG. 2, by selectively increasing an amount of (hot) exhaust gasdirected through EGR cooler 162, heat rejection at the EGR cooler may beincreased. Since the EGR cooler is a heat exchanger configured toexchange with coolant that is fluidly coupled to an engine coolantsystem, the additional heat rejected at the EGR cooler may be used toheat engine coolant, thereby heating the engine. By using this heatrejection to increase an engine temperature during selected operatingconditions, such as during an engine cold-start and warm-up, exhaustcatalyst activation can be expedited while also reducing engine NVHissues experienced during a cold-start. As such, this provides a moreeffective way of recovering latent heat from the water in the exhaust.

A bypass passage 165 may be included in vehicle system 106 to fluidlycouple EGR passage 163 with exhaust passage 145. In particular, bypasspassage 165 may couple EGR passage 163, on the intake passage side ofEGR cooler 162, with exhaust passage 145, downstream of exhaust throttle164 (substantially in tailpipe 135). Bypass passage 163 enables at leasta portion of exhaust gas to be released to the atmosphere upon passagethrough EGR cooler 162. In particular, during conditions when EGR valve159 is closed, exhaust gas (such as throttled exhaust gas generated uponclosing of throttle 164) may be directed into EGR passage 163, then intoEGR cooler 162, and then to tailpipe 135 via bypass passage 165. Byventing some exhaust gas via bypass passage 165 when EGR valve 159 isclosed, an exhaust pressure in EGR passage 163 (upstream of and at EGRcooler 162) can be maintained within limits. As such, this reducesdamage to components of the LP-EGR system. In comparison, duringconditions when EGR valve 159 is open, based on the degree of opening ofEGR valve 159 and exhaust throttle 164, and further based on an amountof EGR requested and a ratio of intake to exhaust manifold pressure,exhaust gas may flow from upstream of exhaust throttle 164 to downstreamof EBV 164, via EGR cooler 162 and bypass passage 165, or fromdownstream of exhaust throttle 164 to the intake passage side of EGRcooler 162 via intermediate passage 165.

In some embodiments (as depicted), an ejector 168 may be arranged inbypass passage 165. A motive flow of exhaust gas through ejector 168 maybe harnessed to generate vacuum at a suction port of ejector 168. Thesuction port of ejector 168 may be coupled with, and stored in, vacuumreservoir 177. The stored vacuum can then be supplied to one or morevehicle system vacuum consumers, such as a brake booster, front endaccessory drive (FEAD), positive crankcase ventilation system,vacuum-actuated valves, etc. A vacuum sensor 192 may be coupled tovacuum reservoir 177 to provide an estimate of available vacuum. In someexamples, exhaust gas may flow from an inlet of ejector 168 (on theintake passage side of the ejector) to an outlet of ejector 268 (on theexhaust passage side of the ejector). In addition to vacuum from ejector168, vacuum reservoir 177 may be coupled with one or more additionalvacuum sources such as other ejectors arranged within vehicle system106, electrically-driven vacuum pumps, engine-driven vacuum pumps, etc.

Depending on the position of exhaust throttle 164 and EGR valve 159,some or all of the exhaust gas exiting catalyst 170 may bypass theexhaust backpressure valve, enter the EGR passage and flow throughbypass passage 165, providing a motive flow through ejector 168. Forexample, when exhaust throttle 164 is open and EGR valve 159 is closed,the exhaust throttle does not restrict exhaust flow through exhaustpassage 145, and little or none of the exhaust flowing in exhaustpassage 145 downstream of catalyst 170 bypasses the exhaust throttle viapassage 165 (depending on the quantity of exhaust flow and relativediameters of passages 145 and 165). When the exhaust throttle ispartially open and the EGR valve is closed, depending on the quantity ofexhaust flow and relative diameters of passages 145 and 165, someexhaust may flow around the exhaust throttle while the remainder of theexhaust is diverted through ejector 168 via passage 165, bypassing theexhaust throttle. When the exhaust throttle is fully closed and the EGRvalve is closed, all exhaust flow is directed into passage 165. When theEGR valve is open, based on the opening of the EGR valve, at least aportion of the exhaust gas exiting catalyst 170 may bypass the exhaustbackpressure valve, enter the EGR passage, and be recirculated intointake passage 142. As elaborated with reference to FIG. 4, based onengine heating needs, vacuum needs, and EGR needs, a position of theexhaust throttle and the EGR valve may be adjusted to operating theengine system in one of multiple operating modes. In doing so, EGR andengine heating requirements may be met while also advantageouslygenerating vacuum at exhaust ejector 168.

In some embodiments (as depicted), vehicle system 106 further includes ahigh-pressure EGR (HP-EGR) system 171. HP-EGR system 171 includes an EGRpassage 173 that couples exhaust passage 145, upstream of turbine 154with air intake passage 142, downstream of compressor 152 and upstreamof charge air cooler 184 and intake throttle 182. An EGR cooler 172arranged in EGR passage 173 cools exhaust gas flowing there-through. Aposition of EGR valve 179, located in EGR passage 173 on the intakepassage side of EGR cooler 172, may be adjusted by controller 120 tovary an amount and/or rate of exhaust gas recirculated from the exhaustpassage to the intake passage via the HP-EGR system. In someembodiments, one or more sensors may be positioned within HP-EGR passage173 to provide an indication of one or more of a pressure, temperature,and air-fuel ratio of exhaust gas recirculated through the HP-EGRpassage.

In this way, the depicted system yields both on-demand exhaust heatrecovery and on-demand vacuum generation at the cost of increasedexhaust back pressure (only during demand). As such, there are threefunctions that need exhaust pressure. The first functions is EGR.Specifically, EGR needs a minimum back pressure to flow at the presentengine condition and EGR flow rate demand. Secondly, exhaust heatrecovery needs a certain backpressure to achieve its heat transferobjective. Finally, the ejector needs a given exhaust backpressure toachieve a given pump down rate. The controller uses an arbitrationstrategy that chooses an exhaust backpressure based on the prioritiesand restrictions of the total system enabling the various demands to bemet.

Engine 100 may be controlled at least partially by a control system 140including controller 120 and by input from a vehicle operator via aninput device (not shown). Control system 140 is configured to receiveinformation from a plurality of sensors 160 (various examples of whichare described herein) and sending control signals to a plurality ofactuators 180. As one example, sensors 160 may include exhaust gasoxygen sensor 126 coupled to exhaust manifold 148, MAP sensor 121coupled to intake manifold 144, exhaust catalyst temperature sensor 117,exhaust pressure sensor 119 located upstream of catalyst 170 in tailpipe135, exhaust temperature sensor 128 and exhaust pressure sensor 129located downstream of catalyst 170 in tailpipe 135, and vacuum sensor192 arranged in vacuum reservoir 177. Various exhaust gas sensors mayalso be included in exhaust passage 145 downstream of catalyst 170, suchas particulate matter (PM) sensors, NOx sensors, oxygen sensors, ammoniasensors, hydrocarbon sensors, etc. Other sensors such as additionalpressure, temperature, air/fuel ratio and composition sensors may becoupled to various locations in the vehicle system 106. As anotherexample, actuators 180 may include fuel injector 166, exhaust throttle164, EGR valve 159, and intake throttle 182. Other actuators, such as avariety of additional valves and throttles, may be coupled to variouslocations in vehicle system 106. Controller 120 may receive input datafrom the various sensors, process the input data, and trigger theactuators in response to the processed input data based on instructionor code programmed therein corresponding to one or more routines.Example control routines are described herein with regard to FIGS. 2 and4.

Now turning to FIG. 2, routine 200 depicts a method for adjusting theposition of an EGR valve (such as an EGR valve in a low-pressure EGRsystem) and an exhaust back-pressure valve during an engine cold-startand warm-up to expedite catalyst heating and activation while alsoaddressing engine cold-start NVH issues.

At 202, the routine includes confirming an engine cold-start. Forexample, it may be determined if an engine temperature (e.g., asinferred from an engine coolant temperature) is below a threshold. Inanother example, it may be determined if a temperature at an exhaustcatalyst is below a threshold temperature, such as below an activationor light-off temperature. If not, the routine may end.

Upon confirming an engine cold-start, at 203, the routine includesclosing a post-catalyst exhaust backpressure valve (or exhaustthrottle). In one example, closing the post-catalyst exhaust throttleincludes fully closing the throttle. In another example, closing thepost-catalyst exhaust throttle includes moving the exhaust throttle fromthe current position to a more closed position. By closing the exhaustthrottle, an exhaust backpressure may be increased, thereby increasingan exhaust temperature, which assists in expediting exhaust catalystheating. In addition, while the temperature of the exhaust catalyst isbelow the threshold temperature, and while the exhaust throttle isclosed, the routine includes retarding ignition spark timing. Byretarding spark timing, the exhaust temperature may be furtherincreased, further assisting in expediting exhaust catalyst heating. Anamount of spark retard applied may be adjusted based on the temperatureof the exhaust catalyst. For example, as a difference between theexhaust catalyst temperature and the threshold temperature increases,more spark retard may be applied (as long as combustion stability is notdegraded).

At 204, the routine includes closing an EGR valve. In one example,closing the EGR valve includes fully closing the EGR valve. In anotherexample, closing the EGR valve includes moving the EGR valve from thecurrent position to a more closed position. Herein, the EGR valve may becoupled in an LP-EGR system (such as LP-EGR system 161 of FIG. 1). Byclosing the EGR valve while also closing the exhaust throttle, at leasta portion of the throttled exhaust gas is diverted through an EGR coolerof the LP-EGR system. In particular, a portion of the throttled exhaustgas is diverted through an EGR cooler located inside an EGR passagewhile maintaining an EGR valve in the EGR passage at a more closedposition, wherein the EGR passage fluidly couples the engine exhaustfrom upstream of the exhaust throttle and downstream of the exhaustcatalyst to an engine intake, upstream of an intake compressor. Herein,the EGR passage is a low pressure EGR passage. In other words, a largerportion of catalyst-treated exhaust gas is taken off from upstream ofthe exhaust throttle and diverted through an EGR cooler. Since thethrottled exhaust gas has a higher temperature, passage of the hotexhaust gas through the EGR cooler causes a rise in heat rejection atthe EGR cooler. Since the EGR cooler is coupled to the engine coolantsystem, the rejected heat is advantageously used to heat up the engineand the exhaust catalyst during the cold-start. Thus, the synergisticcombination of exhaust gas throttling and increased heat rejection atthe EGR cooler can be used to expedite exhaust catalyst activationfaster than can be achieved with either method alone. Further, by usingexhaust throttle and EGR valve adjustments concomitantly, the exhausttemperature can be increased while using a smaller amount of sparkretard, thereby providing fuel economy benefits during the enginecold-start.

The diverting further includes routing the portion of throttled exhaustgas from an outlet of the EGR cooler to the engine exhaust, downstreamof the exhaust throttle, via a bypass passage. The exhaust gas may thenbe vented to the atmosphere. In some embodiments, the bypass passage mayinclude an ejector. In those embodiments, the portion of throttledexhaust routed through the bypass passage may be flowed through theejector, enabling vacuum to be drawn at the ejector. In this way, thethrottled exhaust flow through the EGR cooler can be used to expeditecatalyst heating while additionally generating vacuum. The generatedvacuum may then be used for the actuation of one or more engine vacuumactuators (e.g., a brake booster) that are coupled to the ejector.

At 206, while operating with the exhaust throttle and EGR valve closed,it may be determined if an exhaust backpressure is higher than athreshold pressure. In one example, the exhaust back-pressure may beestimated at a location upstream of the exhaust throttle and downstreamof the catalyst (e.g., by a dedicated pressure sensor). In otherexamples, the exhaust backpressure may be inferred based at least on thetemperature of the exhaust gas and a position (or degree of closing) ofthe exhaust throttle. As such, the closing of the exhaust throttle canlead to an increase in exhaust backpressure (and temperature) which isused to heat the catalyst. However, if the exhaust backpressure risestoo far, engine component damage may occur (e.g., damage to the exhaustcatalyst). Thus, at 208, while the temperature of the exhaust catalystis below the threshold temperature, the routine includes intermittently(or transiently) opening the exhaust throttle in response to the exhaustback-pressure estimated upstream of the throttle (and downstream of thecatalyst) being higher than a threshold pressure. Upon relieving theexhaust backpressure, the routine proceeds to 210 from 208. Else, if noexhaust backpressure build-up has occurred, the routine directlyproceeds to 210 from 206.

As such, the closing of the valve and exhaust throttle and the divertingof exhaust gas through the EGR cooler is performed for a duration untila temperature of the exhaust catalyst is above a threshold temperature.For example, it may be continued until the exhaust catalyst has beensufficiently activated. Accordingly, at 210, it is determined if theexhaust temperature (or exhaust catalyst temperature) is at or above athreshold temperature, such as a catalyst light-off temperature(T_lightoff). If not, at 212, the exhaust throttle and the EGR valve maybe maintained in the more closed position until the catalyst temperatureis sufficiently high.

As such, even after the catalyst has been sufficiently heated, theengine may not be sufficiently heated, leading to NVH issues at theengine cold-start. Thus, even after the catalyst is activated, theexhaust throttle and the EGR valve may be maintained closed to continuerejecting heat at the EGR cooler to heat up the engine (via heating ofengine coolant). Accordingly, after the temperature of the exhaustcatalyst is above the threshold temperature, at 214, the routineincludes maintaining the exhaust throttle closed and the EGR valveclosed while advancing spark ignition timing (or reducing an amount ofspark retard). Herein spark timing may be advanced to decrease exhaustheating via the spark timing adjustment. Advancing spark timing mayinclude advancing the spark ignition timing from the retarded timingthat was set at 203 towards an original timing that was set before 203.Alternatively, spark timing may be advanced (or spark retard may bereduced) to a timing based on the prevalent engine operating conditions.

Next, at 216, an EGR cooler outlet temperature may be estimated and itmay be determined if the EGR outlet temperature is higher than athreshold temperature. The EGR cooler outlet temperature may beestimated, for example, by a temperature sensor coupled in the EGRpassage, downstream of EGR cooler (such as sensor 118 of FIG. 1). In oneexample, the threshold temperature may correspond to a temperature at orabove which the engine may be sufficiently warm and NVH issues may bereduced. As such, the exhaust throttle and EGR valve may be maintainedclosed until the EGR cooler outlet temperature is sufficiently warmed.

At 218, after the EGR outlet temperature has risen above the thresholdtemperature, the exhaust throttle may be opened (or moved to a more openposition). In an alternate example, after the temperature of the exhaustcatalyst is above the threshold temperature, the exhaust throttle may beadjusted based on the EGR cooler outlet temperature with the exhaustthrottle shifted from a more closed position to a more open position asthe outlet temperature of the EGR cooler increases.

At 220, after the EGR outlet temperature has risen above the thresholdtemperature, the EGR valve may also be opened (or moved to a more openposition) if EGR is required. In particular, an opening of the EGR valvemay be adjusted based on the engine's EGR (and engine dilution)requirement.

In this way, during an engine during an engine cold-start and warm-up,an engine may be restarted with each of a post-catalyst exhaust throttleand an EGR valve closed. With the valves closed, at least a portion ofthrottled exhaust gas may be diverted around the exhaust throttle via anEGR cooler and an ejector. Each of the exhaust throttle and the valvemay then be maintained closed until each of an exhaust temperature andan EGR cooler outlet temperature is above a threshold. This not onlyexpedites exhaust catalyst activation but also reduced engine cold-startNVH issues. Thus, by using an increase in exhaust backpressure andincrease in heat rejection at the EGR cooler, synergistic benefits areachieved in that each of an engine temperature and an exhaust catalysttemperature are raised to activation levels faster than would otherwisehave been possible with either an increase in exhaust backpressure or anincrease in heat rejection at the EGR cooler. In addition, the exhaustflow can be opportunistically harnessed for vacuum generation, thevacuum subsequently used for actuating various engine vacuum actuators.By opportunistically generating additional vacuum during the enginecold-start and warm-up, while expediting engine heating and catalystactivation, the need for operating dedicated vacuum pumps for variousvacuum actuators is reduced.

It will be appreciated that FIG. 2 does not mention vacuum production inits decision tree. This is because when operating in a catalyst heatingmode, the ejector is already provided with a high backpressure and thusis providing a significant contribution to vacuum production. However,in alternate embodiments, FIG. 2 may further include vacuum productionin the decision tree.

Coordination of exhaust backpressure valve and EGR valve adjustments toexpedite catalyst activation while raising engine temperature is nowshown with reference to the example of FIG. 3. Specifically, map 300depicts an EGR cooler outlet temperature at graph 301, an exhaustcatalyst temperature at graph 302, an exhaust backpressure at graph 304,exhaust throttle adjustments at graph 306, and spark timing adjustmentsat graph 308. All graphs are plotted against time (along the x-axis).

At t1, an engine may be started and warmed-up. In particular, inresponse to the engine catalyst temperature (302) being below athreshold (T_light-off), an engine cold-start may be initiated at t1.During the engine cold-start, the engine is operated with each of apost-catalyst exhaust throttle (306) and an EGR valve (not shown)closed. In the depicted example, the exhaust throttle and the EGR valveare fully closed, however it will be appreciated that in alternateexamples, the exhaust throttle and the EGR valve may be moved to a moreclosed position. Closing the exhaust throttle causes an exhaustbackpressure estimated upstream (e.g., immediately upstream) of theexhaust throttle to increase (304) as well as the catalyst temperatureto increase (302).

With the exhaust throttle closed, at least a portion of throttledexhaust gas is diverted into an EGR passage (or EGR take-off) includingthe EGR valve and an EGR cooler positioned upstream of the EGR valve. Inthe present example, each of the EGR valve and the EGR cooler may bepositioned in a low pressure EGR passage, the EGR passage fluidlycoupling an engine exhaust, from upstream of the exhaust throttle anddownstream of the catalyst to an engine intake, upstream of an intakecompressor. The increased flow of heated exhaust gas through the EGRcooler causes a rise in temperature at the EGR cooler (as shown by anincrease in EGR cooler outlet temperature, 301). This in turn causesincreased heat rejection at the EGR cooler, the heat rejected to theengine coolant. The heated coolant then leads to an increase in enginetemperature which helps to reduce engine NVH issues at cold-start whilealso assisting in heating the exhaust catalyst. With the EGR valve alsoclosed, the heated exhaust gas diverted through the EGR cooler is thenflowed from the EGR cooler outlet into a bypass passage which connectsback to the engine exhaust, downstream of the exhaust throttle. Fromthere, the exhaust gas is vented to the atmosphere. As such, thecombination of closing the exhaust throttle and the EGR valve (toincrease exhaust backpressure and temperature and heat rejection at theEGR cooler) expedites catalyst heating. In particular, as depicted, theapproach enables the catalyst temperature to reach the threshold(T_lightoff) in a smaller amount of time than would be possible withoutclosing both valves (the claimed approach initiated at t1 enables thecatalyst temperature to reach the threshold at t2, while in the absenceof the claimed approach, catalyst temperature would reach the thresholdat t4, as depicted by dotted graph 303).

In some embodiments, where the bypass passage includes an ejector, theexhaust gas is diverted around the exhaust throttle via the EGR coolerand the ejector. When the ejector is included, exhaust flow through theejector may be harnessed enabling vacuum to be drawn at a neck of theejector, the drawn vacuum then provided to one or more vacuum consumersof the engine (such as for operating a brake booster, for purging acanister, for crankcase ventilation, etc.)

During the warm-up, while the EGR cooler outlet temperature and thecatalyst temperature are below their respective thresholds, and whilethe exhaust throttle and the EGR valve are closed (between t1 and t2),spark ignition timing may be retarded from MBT (308). In particular, anamount of spark retard applied may be increased to further increaseexhaust temperatures and further expedite catalyst activation.

At t2, the catalyst temperature may be above threshold temperatureT_lightoff but the EGR cooler outlet temperature may still be below thedesired threshold (T_egrcot). Thus, after t2, each of the exhaustthrottle and the valve may be maintained closed until the EGR cooleroutlet temperature is above the threshold. In addition, after thecatalyst has reached the threshold temperature, and while the EGR cooleroutlet temperature is increasing towards the threshold, an amount ofspark retard from MBT that is applied may be reduced. That is, sparkignition timing may be advanced towards (or returned towards) MBT. Inone example, ignition timing may be returned to the original settingused before t1.

Between t2 and t3, the EGR cooler outlet temperature reaches thresholdT_egrcot. Thus, between t2 and t3, as the EGR cooler outlet temperatureincreases above the threshold, the exhaust throttle is moved to a moreopen position. This allows an exhaust backpressure and temperature toreduce. In addition a larger portion of the catalyst-treated exhaust gasis vented to the atmosphere through the exhaust throttle while only asmaller, remaining portion is vented to the atmosphere around thethrottle, upon flowing through the EGR cooler and the bypass passage.Consequently, soon after t2, the EGR outlet temperature continues toincrease for a short while, but then as the exhaust throttle is openedand the exhaust temperature drops, the EGR outlet temperature alsostarts to fall and stabilize at a lower value. It will be appreciatedthat while the depicted example shows the exhaust throttle beinggradually moved to a more open position after t2, in alternateembodiments, the exhaust throttle may be fully opened at t2.

While the EGR cooler outlet temperature is below the threshold, theexhaust throttle may be intermittently opened in response to an exhaustbackpressure upstream of the exhaust throttle being above a thresholdpressure. For example, as shown at 305, in response to exhaustbackpressure (304) rising above threshold T_ebp, the exhaust valve maybe transiently opened to relieve the backpressure.

Optionally, the EGR valve may be opened after the exhaust throttle hasbeen opened to provide a desired amount of exhaust gas recirculation. Assuch, the EGR amount required may be determined based on engineoperating conditions and engine dilution requirements. For example, ifmore engine dilution is required, the EGR valve may be moved to a moreopen position.

In some embodiments, the various exhaust throttle and EGR valveadjustments may be performed to operate an engine system in variousmodes. In one example, an engine system may comprise an engine includingan intake and an exhaust, a turbocharger including an intake compressorand an exhaust turbine, an exhaust catalyst, and a post-catalyst exhaustthrottle. The system may further include an EGR system including an EGRpassage, an EGR cooler and an EGR valve, the system fluidly coupling theengine exhaust downstream of the catalyst and upstream of the exhaustthrottle, to the engine intake, upstream of the compressor. A branch orbypass passage including an ejector may fluidly couple an outlet of theEGR cooler to the engine exhaust, downstream of the exhaust throttle.The engine system may further include a controller with computerreadable instructions for operating the system in the various modes. Forexample, the engine system may be operated in a first mode with each ofthe exhaust throttle and the EGR valve closed and while flowing exhaustgas from the catalyst, through the EGR cooler and then through theejector. As another example, the system may be operated in a second modewith each of the exhaust throttle and the EGR valve open and whileflowing exhaust gas from the catalyst, through the ejector and thenthrough the ejector. During both the first and second modes, vacuum maybe drawn at the ejector. The controller may operate the system in thefirst mode during conditions when the exhaust catalyst is below athreshold temperature, while operating the system in the second modeduring conditions when the exhaust catalyst is above the thresholdtemperature and an engine vacuum requirement is higher than a threshold.In some embodiments, while operating in the first mode, ignition sparktiming may be retarded by a higher amount. In comparison, whileoperating in the second mode, ignition spark timing is retarded by asmaller amount.

Now turning to FIG. 4, an example routine 400 is shown for adjusting anEGR valve and an exhaust throttle in concert so to maintain an amount ofEGR flow to an engine while meeting vacuum needs of various enginevacuum consumers.

At 402, the method includes estimating engine operating conditions. Thismay be include measuring and/or inferring conditions such as enginetemperature, exhaust temperature and pressure, barometric pressure,engine speed, boost level, manifold pressure, manifold air flow, etc. At404, based on the estimated operating conditions, an EGR requirement ofthe engine may be determined. For example, an amount of engine dilutionor residuals required to improve engine performance and combustionstability may be determined. Based on the determined EGR requirement, anEGR valve position may be determined. In particular, an opening of theEGR valve may be determined based on the EGR requirement, the EGR valveopening increased (that is, the EGR valve shifted to a more openposition) as the EGR requirement increases.

At 406, it may be determined if an exhaust catalyst temperature ishigher than a threshold temperature, such as above a light-offtemperature. In other words, it may be determined if any EGR valve andexhaust throttle adjustments are required to expedite catalyst heating.If the catalyst is sufficiently hot and activated, then at 410, theroutine includes determining if an engine vacuum requirement is higherthan a threshold. For example, it may be determined if there is atransient increase in vacuum requirement due to actuation of one or moreengine system vacuum consumers/actuators. In an alternate example, itmay be determined if manifold vacuum needs to be supplemented to enableactuation of the various engine system vacuum consumers.

At 412, if there is no (additional) vacuum requirement, the routineincludes operating the engine system in a first EGR mode (Mode_EGR1)with the exhaust throttle more open to recirculate a portion of exhaustgas to the engine intake while directing a first, smaller amount ofexhaust gas through the EGR cooler and then through the ejector. Herein,while recirculating the desired amount of exhaust gas to the engineintake, a smaller amount of catalyst-treated and throttled exhaust gasis vented to the atmosphere upon passing through the EGR cooler (wheresome heat is rejected), and then through the ejector, and then to theexhaust passage, downstream of the exhaust throttle. At least somevacuum is potentially drawn at the ejector when operating in this firstmode, the vacuum generated at the neck of the ejector due to flow ofexhaust gas there-through. As such, when exhaust flows through theejector, the potential to create a suction flow exists. However actualflow may depend on the ejector conditions and vacuum reservoir pressure.When operating in the first EGR mode with the exhaust throttle moreopen, the EGR valve may also be more open, with the opening of the EGRvalve based on the portion of exhaust gas recirculated to the engineintake. That is, the EGR valve may be shifted to a more open positionwith the more open position selected to meet the engine's EGRrequirement.

In comparison, at 414, if there is a vacuum requirement, the routineincludes operating the engine system in a second EGR mode (Mode_EGR2)with the exhaust throttle more closed to recirculate the portion ofexhaust gas to the engine intake while directing a second, larger amountof exhaust gas through the EGR cooler and then through the ejector. Theclosing of the exhaust throttle may be performed, for example, inresponse to actuation of a vacuum actuator, or in response to areduction in measured/inferred vacuum in a vacuum reservoir coupled tothe vacuum actuator. Herein, by closing the exhaust throttle, a largeramount of exhaust gas may be diverted into the EGR passage. The higherbackpressure generated upstream of the exhaust throttle may cause EGRdisturbances due to an increase in EGR flow due to the closing of theexhaust throttle. Thus, when operating in the second EGR mode, the EGRvalve may be more closed, the closing of the EGR valve based on theclosing of the exhaust throttle to maintain recirculation of the portionof exhaust gas to the engine intake. For example, as the exhaustthrottle is moved to a more closed position, the EGR valve may also bemoved to a more closed position to maintain the portion of EGR providedto the engine intake. Herein, while recirculating the desired amount ofexhaust gas to the engine intake, a larger amount of catalyst-treatedand throttled exhaust gas is vented to the atmosphere upon passingthrough the EGR cooler (where more heat is rejected), and then throughthe ejector, and then to the exhaust passage, downstream of the exhaustthrottle. As in the first mode, at least some vacuum is potentiallydrawn at the ejector when operating in the second mode, the vacuumgenerated at the neck of the ejector due to flow of exhaust gasthere-through. However, a larger amount of vacuum is drawn at theejector when operating in the second EGR mode as compared to the firstEGR mode due to the larger flow of exhaust through the ejector in thesecond mode. In both the first and second EGR modes, the vacuum drawn atthe ejector may be used by one or more engine vacuum actuators coupledto the ejector.

In some embodiments, the controller may monitor an exhaust temperatureduring the first or second modes. The controller may also monitor anexhaust backpressure estimated upstream of the exhaust throttle duringthe operating modes. In particular, when reducing an opening of theexhaust throttle valve during the second mode, there may be an increasein exhaust backpressure and exhaust temperature. Therein, in response tothe exhaust temperature rising above a threshold temperature, thecontroller may increase the opening of each of the exhaust throttle andthe EGR valve to being the exhaust temperature within a desired range.Likewise, in response to the exhaust backpressure rising above athreshold pressure, the opening of each of the exhaust throttle and theEGR valve may be increased to reduce the backpressure. In one example,adjustments performed during the first or second modes that are based onan increase in exhaust temperature and/or back-pressure may be transientchanges. Therein, once the pressure and temperature are within thedesired range, the original settings for the throttle and the EGR valvemay be resumed.

It will be appreciated that while the depicted routine shows an enginecontroller selecting an operating mode of the engine system based on thevacuum requirement, in an alternate embodiment, the controller may beconfigured to transition the engine system from operating in the firstmode to operating in the second mode in response to an increase invacuum requirement by the one or more engine vacuum actuators. Forexample, the first mode may be a default EGR operating mode and thecontroller may shift the engine system to the second EGR operating modeto maintain EGR while also using the exhaust flow to generate vacuum formeeting the engine's vacuum need.

Returning to 406, if the catalyst is not sufficiently hot or activated,such as during an engine cold-start and warm-up, then at 408, theroutine includes operating the engine system in a third non-EGR mode(Mode_(—)3) with each of the exhaust throttle and the EGR valve fullyclosed to direct a third amount of exhaust gas through the EGR coolerand then through the ejector. Herein, by closing the exhaust throttle, alarger amount of exhaust gas may be diverted into the EGR passage whilethe higher backpressure generated upstream of the exhaust throttle maybe used to heat the exhaust and expedite catalyst activation. At thesame time, by closing the EGR valve, the heated exhaust gas divertedinto the EGR passage may be forced to flow through the EGR cooler andthen through the ejector before being vented to the atmosphere. Byflowing heated exhaust gas through the EGR cooler, more heat may beexchange by the EGR cooler with the engine coolant system, allowing anengine temperature to increase. This synergistically allows the engineexhaust to be heated and further expedites catalyst activation. Thus,the third amount of exhaust gas passed through the EGR cooler in thethird mode may be higher than each of the first and second amounts,although the third amount may not be recirculated to the engine intake.

While the depicted routine shows an engine controller operating theengine system in the third mode in response to an engine temperaturebeing lower than a threshold temperature, in further embodiments, thecontroller may transition the engine system from the third mode to thefirst mode in response to the exhaust temperature and/or the enginetemperature being higher than the (respective) threshold temperatures.When transitioning from the third mode to the first mode, an opening ofthe exhaust throttle may be increased as EGR cooler outlet temperatureincreases, while an opening of the EGR valve may be increased as engineEGR requirement increases. In still further embodiments, the controllermay transition the engine system from the third mode to the second modein response to the exhaust temperature and/or the engine temperaturebeing higher than the (respective) threshold temperatures and anincrease in engine vacuum requirement.

Overall, by selecting one of the different engine operating modes, thevarious engine requirements can be met. In general, each of vacuumgeneration, exhaust heat recovery, and EGR need a certain level ofexhaust backpressure to achieve their target. By gathering all theserequirements and providing a target exhaust backpressure that is highenough to serve all three purposes without exceeding pressure ortemperature limits, an engine controller can meet all the demands in anoptimized manner.

In this way, an engine may be operated with exhaust gas recirculation,and in response to a vacuum requirement, each of an exhaust throttle andan EGR valve may be adjusted, in concert, to meet the vacuum requirementwhile maintaining the exhaust gas recirculation. As used herein,operating the engine with exhaust gas recirculation includesrecirculating the amount of catalyst-treated exhaust gas from upstreamof the exhaust throttle to an engine intake via an EGR passage, the EGRpassage including the EGR cooler upstream of the EGR valve. By reducingan opening of each of a post-catalyst exhaust throttle and an EGR valveto increase exhaust flow through an EGR cooler and then through anexhaust ejector, vacuum can be drawn vacuum at the ejector to meet thevacuum need. By reducing the opening of the exhaust throttle based onthe vacuum requirement while reducing the opening of the EGR valve basedon the reduction in opening of the exhaust throttle, an amount ofexhaust gas recirculation can be maintained. By maintaining the desiredengine dilution, engine performance and combustion stability is notdegraded while the vacuum need is also met.

Coordination of exhaust backpressure valve and EGR valve adjustments toprovide EGR while also meeting vacuum needs is now shown with referenceto the example of FIG. 5. In particular, FIG. 5 reflects the fact thatto achieve the target EGR flow rate, one needs extra exhaust backpressure provided by the exhaust throttle. Map 500 depicts an EGR amountat plot 502, an ejector vacuum at plot 504, EGR valve adjustments atplot 506, and exhaust throttle adjustments at graph 508. All graphs areplotted against time (along the x-axis).

Prior to t1, the engine may be operating with no EGR requested.Accordingly, the engine may be operated with the EGR valve closed (506).The exhaust throttle may remain open to allow exhaust gas to be ventedto the atmosphere through the exhaust throttle. At t1, an engine EGRrequirement may increase (dotted line 501). In particular, an amount ofEGR may be requested so as to provide engine dilution. To provide thedesired engine dilution, the EGR valve may be (gradually) shifted to amore open position (506), where the more open position is based onengine operating conditions. By opening the EGR valve, a desired amountof exhaust gas can be recirculated to the engine intake (solid line502). As elaborated with reference to FIG. 1, the EGR valve may beincluded in an EGR passage, or EGR take-off, coupling the engineexhaust, upstream of the exhaust throttle, to the engine intake.Further, the EGR passage may include an EGR cooler coupled downstream ofa juncture of the EGR passage and the engine exhaust and upstream of theEGR valve. Thus, when recirculating, the desired amount ofcatalyst-treated exhaust gas may be diverted from upstream of theexhaust throttle into the EGR passage where it may flow through the EGRcooler and then through the (open) EGR valve before being recirculatedinto the engine intake, upstream of an intake compressor.

At t2, there may be an increase in engine vacuum requirement (dottedline 503). In one example, the increase in engine vacuum requirement maybe due to the actuation of one or more vacuum consumers, such as vehiclebrakes. In response to the increase in vacuum requirement, at t2, thepost-catalyst exhaust throttle may be closed (or moved to a more closedposition) so as to increase vacuum generation (solid line 504) at anexhaust ejector. As elaborated with reference to FIG. 1, the exhaustejector may be located in a bypass passage coupling the EGR passage,downstream of the EGR cooler, to the engine exhaust, downstream of thethrottle. Thus, by closing the post-catalyst exhaust throttle, a portionof the amount of exhaust gas may be flowed through the EGR cooler andthen through the exhaust ejector. That is, catalyst-treated exhaust gasmay be diverted from upstream of the exhaust throttle into the EGRpassage from where a portion of it may flow through the EGR cooler andthen through the ejector before being returned to the engine exhaust,downstream of the exhaust throttle, while the remaining portion flowsthrough the EGR valve into the engine intake. Since the ejector iscoupled to a vacuum actuator, the closing of the exhaust throttle may beperformed in response to actuation of the vacuum actuator. Vacuum isthen generated by the exhaust flowing through the ejector (504), and thegenerated vacuum can be drawn from a neck of the ejector and consumed bythe various engine vacuum actuators.

Closing of the exhaust throttle, however, causes an increase in exhaustbackpressure immediately upstream of the throttle. Since this is thelocation from where EGR is taken off, in the absence of any EGR valveadjustments, the increase in exhaust backpressure can lead to anincrease in EGR recirculated to the engine intake. As such, these EGRfluctuations can degrade engine combustion stability and performance.Thus, also at t2, to maintain the amount of exhaust gas recirculation(at the desired level), the EGR valve may also be closed (or moved to amore closed position). In other words, the closing of the post-catalystexhaust throttle is adjusted based on the engine vacuum need while theclosing of the EGR valve is adjusted based on the closing of the exhaustthrottle. As used herein, closing the EGR valve includes shifting theEGR valve from the more open position (at t2) to a more closed position,the more closed position based on the exhaust throttle closing. Whilethe depicted example shows moving the exhaust throttle to a more closedposition in response to the vacuum requirement, and moving the EGR valveto a more closed position in response to the exhaust throttle closing(to maintain EGR), in alternate examples, closing the exhaust throttlemay include fully closing the exhaust throttle, while closing the EGRvalve includes fully closing the EGR valve.

Between t2 and t3, the exhaust throttle and the EGR valve may bemaintained at the more closed positions to continue generating vacuum(504) to meet the vacuum requirement (503) while also providing EGR(502) to meet the EGR requirement (501). At t3, in response to a drop invacuum requirement, the exhaust throttle may be shifted back to a moreopen position. The drop in exhaust backpressure, consequently EGR flow,is compensated for by correspondingly and concomitantly opening the EGRvalve, so that EGR is maintained after throttle opening at t3.

At t4, based on the prevalent engine operating conditions, the EGRrequirement may reduce. In response to the drop in EGR requirement, att4, the EGR valve may be adjusted to a more closed position so as toreduce the amount of exhaust gas diverted from upstream of the exhaustthrottle to the engine intake. The EGR valve may then be maintained atan opening based on the desired engine dilution and other engineoperating conditions.

In this way, exhaust throttle and EGR valve adjustments may becoordinated during various engine operating conditions to provide EGR,expedite heating, and provide vacuum, as needed. During an enginecold-start and warm-up, an engine may be restarted with each of apost-catalyst exhaust throttle and an EGR valve closed. By divertingthrottled exhaust gas through an EGR cooler, an increase in exhaustbackpressure can be used to elevate exhaust temperatures, whileincreased heat transfer at the EGR cooler is synergistically used tofurther expedite catalyst activation as well as reduce engine cold-startNVH issues. By also directing the diverted throttle exhaust gas throughan exhaust ejector, the exhaust flow can be opportunistically harnessedfor vacuum generation. During non-cold-start conditions, exhaustthrottling can be advantageously used to enhance vacuum production whileproviding EGR. By closing the exhaust throttle and diverting moreexhaust gas through the ejector, vacuum needs can be met. Whilesimultaneously closing an EGR valve, a desired engine dilution can bemaintained. Overall, vacuum production can be provided without causingEGR fluctuations, and therefore without degrading engine performance.

Note that the example control and estimation routines included hereincan be used with various engine and/or vehicle system configurations.The specific routines described herein may represent one or more of anynumber of processing strategies such as event-driven, interrupt-driven,multi-tasking, multi-threading, and the like. As such, various acts,operations, or functions illustrated may be performed in the sequenceillustrated, in parallel, or in some cases omitted. Likewise, the orderof processing is not necessarily required to achieve the features andadvantages of the example embodiments described herein, but is providedfor ease of illustration and description. One or more of the illustratedacts or functions may be repeatedly performed depending on theparticular strategy being used. Further, the described acts maygraphically represent code to be programmed into the computer readablestorage medium in the engine control system.

1. A method for an engine, comprising: while recirculating an amount ofexhaust gas to an engine intake, closing a post-catalyst exhaustthrottle to increase vacuum generation at an exhaust ejector; andclosing an EGR valve to maintain the amount of exhaust gasrecirculation.
 2. The method of claim 1, wherein the closing apost-catalyst exhaust throttle is based on an engine vacuum need andwherein the closing an EGR valve is based on the closing of the exhaustthrottle.
 3. The method of claim 2, wherein the EGR valve is included inan EGR passage coupling an engine exhaust, upstream of the exhaustthrottle, to the engine intake, and wherein the EGR passage includes anEGR cooler coupled downstream of a juncture of the EGR passage and theengine exhaust and upstream of the EGR valve.
 4. The method of claim 3,wherein the exhaust ejector is located in a bypass passage coupling theEGR passage, downstream of the EGR cooler, to the engine exhaust,downstream of the exhaust throttle.
 5. The method of claim 4, whereinclosing the post-catalyst throttle to increase vacuum generation at theexhaust ejector includes flowing a portion of the amount of exhaust gasthrough the EGR cooler and then through the exhaust ejector, and drawingvacuum generated by the flowing exhaust at the ejector.
 6. The method ofclaim 5, wherein the ejector is coupled to a vacuum actuator, andwherein the closing of the exhaust throttle is performed in response toactuation of the vacuum actuator.
 7. The method of claim 1, whereinrecirculating an amount of exhaust gas to an engine intake includesshifting the EGR valve to a more open position, the more open positionbased on engine operating conditions, and wherein closing the EGR valveincludes shifting the EGR valve from the more open position to a moreclosed position, the more closed position based on the throttle closing.8. The method of claim 1, wherein closing the EGR valve includes fullyclosing the EGR valve and wherein closing the throttle includes fullyclosing the throttle.
 9. A method for an engine, comprising: operatingthe engine with exhaust gas recirculation; and in response to a vacuumrequirement, reducing an opening of each of a post-catalyst exhaustthrottle and an EGR valve to increase exhaust flow through an EGR coolerand then through an exhaust ejector; and drawing vacuum at the ejector.10. The method of claim 9, wherein the reduction in opening of theexhaust throttle is based on the vacuum requirement and wherein thereduction in opening of the EGR valve is based on the reduction inopening of the exhaust throttle to maintain an amount of exhaust gasrecirculation.
 11. The method of claim 10, wherein operating the enginewith exhaust gas recirculation includes recirculating the amount ofcatalyst-treated exhaust gas from upstream of the exhaust throttle to anengine intake via an EGR passage, the EGR passage including the EGRcooler upstream of the EGR valve.
 12. The method of claim 11, furthercomprising, monitoring an exhaust temperature during the reducing, andin response to the exhaust temperature rising above a thresholdtemperature, increasing the opening of each of the exhaust throttle andthe EGR valve.
 13. The method of claim 11, further comprising,monitoring an exhaust backpressure upstream of the exhaust throttleduring the reducing, and in response to the exhaust backpressure risingabove a threshold pressure, increasing the opening of each of theexhaust throttle and the EGR valve.
 14. An engine system, comprising: anengine including an intake and an exhaust; a throttle downstream of acatalyst in the engine exhaust; an exhaust gas recirculation (EGR)system including an EGR take-off for recirculating catalyst-treatedexhaust gas from upstream of the exhaust throttle to the engine intake,the EGR take-off including an EGR cooler upstream of an EGR valve, and abypass passage coupling an outlet of the EGR cooler with the engineexhaust, downstream of the exhaust throttle, the bypass passageincluding an ejector; and a controller with computer readableinstructions for: operating the engine system in a first EGR mode withthe exhaust throttle more open to recirculate a portion of exhaust gasto the engine intake while directing a first, smaller amount of exhaustgas through the EGR cooler and then through the ejector; and operatingthe engine system in a second EGR mode with the exhaust throttle moreclosed to recirculate the portion of exhaust gas to the engine intakewhile directing a second, larger amount of exhaust gas through the EGRcooler and then through the ejector.
 15. The system of claim 14, whereinwhen operating in the first EGR mode, the EGR valve is more open, theopening of the EGR valve based on the portion of exhaust gasrecirculated to the engine intake, and wherein when operating in thesecond EGR mode, the EGR valve is more closed, the closing of the EGRvalve based on the closing of the exhaust throttle to maintainrecirculation of the portion of exhaust gas to the engine intake. 16.The system of claim 15, wherein the controller includes furtherinstructions for drawing vacuum at the ejector when operating in each ofthe first and second EGR modes, a larger amount of vacuum drawn at theejector when operating in the second EGR mode as compared to the firstEGR mode, the vacuum used by one or more engine vacuum actuators coupledto the ejector.
 17. The system of claim 16, wherein the controllerincludes further instructions for transitioning the engine system fromoperating in the first mode to operating in the second mode in responseto an increase in vacuum requirement by the one or more engine vacuumactuators.
 18. The system of claim 17, wherein the controller includesfurther instructions for: operating the engine system in a third non-EGRmode with each of the throttle and the EGR valve fully closed to directa third amount of exhaust gas through the EGR cooler and then throughthe ejector, the third amount higher than each of the first and secondamount, wherein the engine system is operated in the third mode inresponse to an engine temperature being lower than a thresholdtemperature.
 19. The system of claim 18, wherein the controller includesfurther instructions for transitioning from the third mode to the firstmode in response to the engine temperature being higher than thethreshold temperature.
 20. The system of claim 19, wherein whentransitioning from the third mode to the first mode, an opening of theexhaust throttle is increased as an EGR cooler outlet temperatureincreases, and an opening of the EGR valve is increased as an engine EGRrequirement increases.